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Given the very positive feedback for Dennis Felsing’s tool ghc-vis, which visualizes the heap representation of a Haskell value, including all the gory details such as thunks, values retained by thunks indirections, sharing etc, I saw the need to provide this information also directly in GHCi, without having to load a graphics library or opening extra libraries. So I added the required features (traversing the heap and pretty-printing the results) to my ghc-heap-view package and, also following ghc-vis’s lead, added a ghci file that, when loaded, provides you with a :printHeap command. Here you can see it in action:

A plain value

Prelude> :script /home/jojo/.cabal/share/ghc-heap-view-0.4.0.0/ghci
Prelude> let x = [1..10]
Prelude> x
[1,2,3,4,5,6,7,8,9,10]
Prelude> :printHeap x
_bh [S# 1,S# 2,S# 3,S# 4,S# 5,S# 6,S# 7,S# 8,S# 9,S# 10]

Note that the tools shows us that the list is a list of S# constructors, and also that it is still hidden behind a blackhole. After running System.Mem.performGC, this disappears.

A thunk

Prelude> let x = Just (1 + 1)
Prelude> :printHeap x
Just _bco
Prelude> x
Just 2
Prelude> System.Mem.performGC
Prelude> :printHeap x
Just (S# 2)

Here, we see how the calculation was deferred until forced by showing the value of x. The name _bco stands for a bytecode object as used by the interpreter. Getting useful information from them is a bit harder than for compiled thunks, so for more accurate results put the code in a Haskell source file, compile it and use the GHC.HeapView API to print the interesting parts.

A partial application

Prelude> let a = "hi"
Prelude> let partial = (a ++)
Prelude> partial ""
"hi"
Prelude> System.Mem.performGC
Prelude> let x = (a, partial)
Prelude> :printHeap x
let x1 = "hi"
in (x1,_fun x1)

The information which function is called there (++ in this case) is lost at runtime, but we still see that the second element of the tuple is a partial application of some value to the first element.

A cyclic structure

Prelude> let s = "ho"
Prelude> let x = cycle s
Prelude> length (take 100 (show x))
100
Prelude> System.Mem.performGC
Prelude> :printHeap x
let x0 = C# 'h' : C# 'o' : x0
in x0
Prelude> let y = map Data.Char.toUpper x
Prelude> length (take 100 (show y))
100
Prelude> :printHeap y
C# 'H' : C# 'O' : C# 'H' : C# 'O' : C# 'H' : C# 'O' : C# 'H' : C# 'O' : C# 'H' : C# 'O' : _bh (C# 'H' : C# 'O' : C# 'H' : C# 'O' : C# 'H' : C# 'O' : C# 'H' : C# 'O' : ... : ...)

The cyclic, tying-the-knot structure of cycle is very visible. But can also see how easily it is broken, in this case by mapping a function over the list.

Mutual recursion

Prelude> let {x = 'H' : y ; y = 'o' : x }
Prelude> length (show (take 10 x, take 10 y)) `seq` return ()
Prelude> System.Mem.performGC
Prelude> :printHeap (x,y)
let x1 = C# 'H' : x3
    x3 = C# 'o' : x1
in (x1,x3)

If you want to look at multiple variables at once, just pass a tuple to printHeap

In the hope that this will be a useful tool for you, I uploaded version 0.4.0.0 of ghc-heap-view to hackage.